Amplitude modulation using phased-array antennas

ABSTRACT

An antenna system for producing an amplitude modulated signal at a receiver by properly varying the spatial amplitude distribution of the antenna beam. The antenna beam variation is accomplished by varying the relative phase of a phase-array antenna at the desired modulation rate. A variable DC bias signal is applied to each element of the antenna which is used to steer the beam in angle. An AC signal is also applied to each element which will qppear as AM modulation at a distant receiver.

United States Patent Wright Apr. 23, 1974 AMPLITUDE MODULATION USING3,238,528 3/1966 Hines 343/100 3,600,701 8/1971 Gouldthrope....

PHASED ARRAY ANTENNAS 3,643,075 2/1972 Hayes 343/100 SA [75] Inventor:Maynard Lattimer Wright, San

Jose Calif Primary ExaminerMaynard R. Wilbur [73] Assignee: The UnitedStates of America as Assistant Examiner-Richard E. Berger represented bythe Secretary of the Attorney, Agent, or FirmR. S. Sciascia; Charles-D.B. Navy, Washington, DC. Curry [22] Filed: Oct. 26, 1971 211 Appl. No.:192,410 [57] ABSTRACT An antenna system for producing an amplitudemodulated signal at a receiver by properly varying the spag 343/100 tialamplitude distribution of the antenna beam. The I antenna beam variationis accomplished by varying [58] Fleld of Search 343/100 325/160 therelative phase of a phase-array antenna at the desired modulation rate.A variable DC. bias signal is ap- [56] References C'ted plied to eachelement of the antenna which is used to UMTED STATES PATENTS steer thebeam in angle. An AC signal is also applied 3,697,995 10/1972 Kafitz343/100 SA to each element which will qppear as AM modulation 3,460,1408/1969 Logan 343/100 SA at a distant receiver, 3,238,527 3/1966 Vogt343/100 SA 3,701,156 10/1972 Killion 343/100 SA 9 Claims, 7 DrawingFigures ll BEAM STEERING PHASE CONTROL I9 DC BIAS RF p 1 DRIVER RFDRlVER ,g Zl RF DRIVER 0 DC BIAS SIGNAL AC A MOII'J LLITAIgEION BIASCONTROL 2| r\ r\.r\, RF

DRIVER BEAM AC STEERING MODULATION INPUT CONTROL VATENH'IUAPRZB MI3.806931 SHEET 1 BF 2 /H 15 BEAM STEERING PHASE CONTROL '9 A RF 2IDRIVER 2| RF I 3 DRIVER 2| DRIVER A DC BIAS SIGNAL -Ac MODULATION BIASPHASE CONTROL A 2| r\ r\ r\. RF

DRIVER BEAM AC flR o L I N PQ T ANTENNA APERTURE AMPLITUDE FIG 10 v 80PHASE +|8O FIG 1b m P A PATENTEDAPR 23 1974 3.806; 931

SHEET 2 [1F 2 SRD 33 o DC 5RD yfis BIAS POWER SUPPLY 5RD \37 e fly BEAM5 STEERING CONTROL 4O 39 MOD X 1 4| AC BIAS 0 FIG 30 2 3 9 ref (=0) 4 9mod FIG 0 9 ref (=0) 6 mod FIG... 3C-fi T T T T e 'mod 9 mod 9 mod 9 modAMPLITUDE MODULATION USING PHASED-ARRAY ANTENNAS BACKGROUND OF THEINVENTION 1. Field of the Invention This invention relates generally toan antenna system for producing an amplitude modulated signal at areceiver and more particularly to an antenna system which will producean amplitude modulated signal at a receiver by varying the spatialamplitude distribution of 1 the antenna beam.

2. Description of the Prior Art.

Prior Antenna systems required separate AM modulations whichsubsequently required a complicated RF source. This virtually eliminatedthe use of AM modulation from specialized systems such as ECM systemswhich use phased array antennas.

SUMMARY OF THE INVENTION Briefly, the present invention comprises anantenna system for producing an amplitude modulated signal at a receiverby properly varying the spatial amplitude distribution of the antennabeam. The antenna beam variation is accomplished by varying the relativephase of a phase-array antenna at the desired modulation rate. Avariable DC bias signal is applied to each element of the antenna whichis used to steer the beam in angle. An AC signal is also applied to eachelement which will appear as AM modulation at a distant receiver.

The advantage provided by this unique method and device, which is thesubject matter of the present invention, is that a carrier wave (CW)drive can be used for the antenna RF source. This greatly simplifies thesource and allows the driver to be operated in the more efficientsaturated amplified mode. Moreover, no separate AM modulator is requiredbecause beam steering capability is already present in most phased-arrayantennas at the present time. AM modulation is not used in ECM systemsbecause of the difficulties mentioned above; this new and unique systemwould allow desirable AM modulators to be employed with the present highefficiency components.

STATEMENTS OF THE OBJECTS OF INVENTION A primary object of the presentinvention is to produce an amplitude modulated signal at a receiver byvarying the spatial amplitude distribution of an antenna beam.

Another object of the present invention is to provide a device whichamplifies the RF source and allows the driver of the system to beoperated with a higher average power output.

Another object of the present invention is to provide a device toproduce an amplitude modulated signal at a receiver without a separateAM modulator and provide a continuous amplitude-modulated multipletarget coverage without wasting power.

Another object of the present invention is to provide a device whichallows all of the RF hardware in the phased-array antenna to operate ona continuous fullpower basis.

Other objects and features will be apparent from the followingdescriptions of the invention and from the accompanying drawingswherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of theamplitude modulated phased-array antenna system;

FIG. la is the amplitude modulation and phase excitation waveform for astandard antenna array system;

FIG. lb is an example ofa phase print function generated from theamplitude modulated phased-array an- 0 tenna system illustrated in FIG.1;

FIG. 2 is a schematic diagram of a four element array antenna system;

FIG. 3a is a schematic illustration of the four element phased-arrayantenna illustrated in FIG. 2, in the beam steering mode;

FIG. 3b is a schematic illustration of the four element phased-arrayantenna illustrated in FIG. 2, with a bias signal applied to the thirdand fourth element; and

FIG. 3c is a schematic diagram of the four element array illustrated inFIG. 2, simulating a large aperture operation function.

Referring to FIG. 1, the amplitude modulated antenna system 11 comprisesa phased-array antenna system 13, a beam steering control 15 and amodulation phase control device 17. The phased array antenna system 13further comprises a plurality of antenna elements 19 and a plurality ofRF drivers 21 operatively connected to each element of the plurality ofantenna elements 19. The drivers are used to generate the RF signal toeach antenna. Each RF driver 21 contains its own individualphase-shifting elements, either digital or analog. Moreover, each of thedrivers is connected to a single antenna element. This portion of theantenna system is similar to most phased-array antennas as presentlyconfigured. The unique difference of the present system is theutilization of the spatially selective amplitude modulator (SPASAM)technique, hereinafter referred to as SPASAM, for control of the phasefor each of the RF driver elements. A conventional phasedarray antennais intended to form a sharp beam as illustrated by the solid line inFIG. 1 and is able to steer or point the beam in the direction of targetT. This aformentioned function is accomplished by driving the ensembleof RF driver elements 21 with a linear phase gradient across the antennaaperture. The signal set, required to accomplish this, is generated bythe beam steering control 15. The beam steering control 15 generates aplurality of individual DC bias outputs which are individually appliedto each RF driver. The beam steering control 15 accepts a single inputDC signal and specifies the direction that the beam should be pointedand subsequently generates a DC bias signal for each of the RF driverelements of the plurality of RF driver elements 21 which will shift thephase appropriately to form and point the beam in the desired direction.In most phased-arrays the antenna pattern remains nearly constantas thebeam is steered to different points in space, or when modulation isapplied to the RF going through the antenna. However, when spatialmodulation circuitry is used, a different function results, whichhereinafter will be described. The modulation phase control device 17has a single modulation input. The input is an AC signal which can begenerated by an oscillator circuit or any similar device. The modulationcontrol device 17 then generates an appropriate modulation phase controlsignal or AC bias for each of the RF driver elements. The aforementionedAC and DC ll w bias signals are generated simultaneously. This secondset of phase control or bias signals allows the antenna pattern to bechanged in both space and time according to the applications of theparticular system. The modulated beam is illustrated by the shadedportion in FIG. 1 so that the distant target T will, at any point withinthis shaded beam, receive an amplitude modulated signal.

There are two broad categories of amplitude modulation generation whichcan be used with the amplitude modulated antenna system. The first isthe beamsteering mode and the second is the unique SPASAM technique.

Referring again to FIG. I, assume that the beam steering control isforming and pointing the beam shown by the solid block line toward adistant target T. This function is usually accomplished by a constantphase difference between each of the adjacent RF driver elements 21.

Modulation is generated on the antenna beam by applying a set ofmodulation phase-control signals to each of the RF driver elements 21.In this case, the signal applied to each RF driver element is moderatelysmall compared to the signal being applied to the beamsteering controland thus results in only a small perturbation of the phase of each RFdriver element. If, in addition, the modulator phase control signals areapplied in the same uniform phase difference pattern as the beamsteering control signals, then the spatial power pattern for the antenna19 will remain constant as a function of time. However, the position ofthe entire pattern will move in space at the modulation rate. This iscalled the beam-steering mode because the antenna power pattern remainsconstant in a well formed beam as a function of time. Moreover, it hasbeen found that the relative amplitude modulation of signals will varyas the position of the receiver varies in space.

The spatially selective amplitude modulation technique or SPASAMincludes all the modes of operation that are possible in which theantenna beam shape is not constant as a function of time. Using theSPASAM technique, the antenna may be made to operate as a linearamplitude modulator in which the waveform is identical to that of themodulation signal input, or it is possible to produce signals which havewaveforms different from the modulation signal.

To facilitate the understanding of the SPASAM technique, a discussion ofthe general theory is necessary.

The antenna of any phased-array system 21 is generally specified by anamplitude and a phase function across the antenna aperture, asillustrated in FIG. la. A constant amplitude of unity with no relativephase shift across the aperture is indicated. This relationship wouldproduce the familiar sin x/x distribution from a phased-array antennawith the beam pointed on the broadside. If the phase variation is madelinear across the aperture with a constant slope the sin x/x patternwill remain, but the antenna beam will now be pointed in a differentdirection in space. In the SPASAM function the phase function does notvary linearly across the aperture. This has the effect of producing adifferent antenna pattern as the phase pattern across the aperture ischanged. This additional phase variation across the aperture will becalled, for various technical reasons, the phase print function for theSPASAM technique. FIG. 1b shows an example of a phase print functionconsisting of 4% cycles of sinusoidal phase variation across the antennaaperture. It has been found that the phase print exists only for acertain instant in time and will be a complex function of time. The setofphase print functions over a time T will be called the modulationexcitation function. In order to facilitate the description of the manytypes of operations that are possible in SPASAM, different classes ofmodulation excitation functions have been generally identified by classof operation. A summary of these classes are illustrated in Table I, anda description of the various classes of modulation excitation functionswill follow.

TABLE I CLASSES OF MODULATION EXCITATION FUNCTIONS Class Power PatternZero Phase Shift at Sinusoidal Stays the Same Modulation ExcitationFrequency Time-Waveform 1 Yes Yes Yes 2 No Yes Yes 3 No No Yes 4 No YesNo 5 No No No 6 No No No 7 No No No Class Same Timc- Same FnuricrWaveform at Fundamental Each [Element Frequency 1 Yen You 2 Yes Yes 3Yes Yes 4 Yes Yes 5 Yes Yes 6 No Yes 7 No No As stated previously, theset of phase print functions over a Time T will define the modulationexcitation function. Of the seven classes of modulation excitationfunctions that will be described, only the first, Class 1, does notgenerate SPASAM. The description of these seven classes in Table l is asfollows:

The identifying characteristic of the first class of modulation signalsis that the radiation power pattern shape is kept constant in time. Thedriving signals from the RF drivers 21 may be quasistatic or may be amodulation signal that corresponds to the beam-steering mode ofamplitude modulation, or a combination of the two. The phase print forthis class is the same as the phase pattern used to steer the beam. SeeFIG. 1a.

The second class of modulation excitation function is more complex thanclass one, but is the simplest form of SPASAM because the radiationpower pattern changes as a function of time. The phase print function isnot the same as the steering phase pattern. The signal applied to eachRF phase shifter has the same time waveform and differs only inmagnitude. That is, the signal applied to one phase shifter will bedifferent by a scale factor from the signal applied to another phaseshifter.

It has been found by experimentation, in class three, that themodulation excitation functionflt) is a sinusoidal function. Thefrequency of this f(t) is the modulating frequency. For example, if thef(t) for element 1 of the antenna is lagging, the f(t) for element 2 ofthe antenna by some number of degrees by some number of degrees, thisfact can be expressed as a phase shift by some number of degrees at themodulation frequency between element 1 and element 2. This phase shiftat the modulation frequency forms the basis for this class. Themodulation excitation function in which the signal to each RF driver hasthe same frequency and the same peak amplitude, but a different phaseshift at the same modulation frequency is exemplified by this class.

The fourth class forms a nonsinusoidal, but periodic, time waveform.This class has the same restrictions as class two, but now f(t) can beany nonsinusoidal, nonlinear, but periodic function. The period for allantenna element modulation signals must be the same. In this instance,it is desirable to decompose the signal waveform into its Fouriercomponents.

Class five allows a phase shift at the modulation frequency, asexplained in class three, to occur for the same conditions as classfour.

In class six a different time waveform is applied to each of the RFphase shifters. Again, however, the time waveform is restricted tohaving the same fundamental Fourier period from element to element, butthe amplitude and phase of any or all of the components are allowed tovary from point to point. In class seven the waveform applied to each ofthe RF elements is different and the difference is due to a differentfundamental Fourier component of the modulation signal. This set ofmodulation signals is approaching the maximum decorrelation of thesignal from antenna element to antenna element and results in aspatially complex transmitted signal from the antenna. The limit of thisexample would necessarily be uncorrelated noise applied to each of theRF phase shifters.

Thus, by varying the AC and DC bias signals generated by thebeam-steering control and the modulation phase control 17, we candevelop the various classes of modulation excitation functions,illustrated in Table l, and respective phase print function signals,illustrated in FIG. 1b.

In many applications it is required that the antenna form and pointbeams to more than one point in space simultaneously in order toilluminate multiple targets. It has been found that the SPASAM techniqueallows this, and in addition, will permit each of the beams to producethe desired broadside AM signal at the target. The first class,illustrated in Table I, can perform the aforementioned function. In thiscase the signals driving the RF phase shifters all have the same timewaveform. It should be noted that this is not beam-steering modulation.The static pattern is not being swept back and forth. The beams at thedesired location are made to grow and shrink by means of atime-modulated phase print function; that is, a modulation excitationfunction. Power taken out of the main beam is put into modulated beamsto produce these types of signals.

The classes two through seven may be used for modulation excitationfunctions and the multiple target problem. This group of modulationexcitation functions allows a trade-off between maximum peak power andmultiple target efficiency, and it is the simplest of the group ofmodulating functions compatible with a multiple target environment.

An example of an embodiment of the present invention, which has beenfound to be quite satisfactory, is illustrated in H6. 2.

Referring to FIG. 2, an amplitude modulated signal is produced at thereceiver by properly varying the signal to the relative element phasesin a phased-array antenna at the desired modulation rate. The antennasystem 31 comprises a linear array of four elements 33, 35, 37, and 39with non-uniform spacing and single large reflecting plane 41. The fourelements 33, 35, 37,

and 39 are individual dipole elements. The antenna beam variation isaccomplished by varying the relative phases of the phase-array antenna31. The phase of the 3GHZ constant signal from each element can beindividually voltage controlled. However, it should be noted that anymethod of individually controlling of each element is workable. Appliedto each antenna element 33, 35, 37, and 39 is a variable DC bias signalwhich is used to steer the beam in angle and an AC signal which willappear as AM modulation at a distant receiver. The array 31 may bedriven by a single RF oscillator and power divider, or other similardevices.

More specifically, elements 33 and 35 are each paired and spaced by awavelength and the other two elements 37 and 39 are separated by '25wavelengths. Element 33 isspaced 3% wavelengths from element 37. Thisspacing is chosen only to illustrate a point and not necessarily foroperation. This particular configuration simulates a single antennawhich is filled in over the entire four wavelength aperture. Thereflecting plane 41 may be constructed of copper or an equivalent metalwith similar electrical characteristics. A step recovery diode (SRD), orany similar frequency multiplier, can be used for the generation ofS-band signals and the phase-shifter. Each element should be adjusted tohave a voltage standing wave ratio (VSWR) of less than about 1.05 at anoperating frequency of about 3 Ge. Each element is fed by a steprecovery diodemodule (srd) that can supply about several milliwatts ofpower. The four step recovery diode (SRD) modules in turn are suppliedfrom a two-watt oscillator or amplifier, as the case may be, and afour-way power divider. A bias supply feeds each module to allow for theindividual and simultaneous application of DC bias, AC bias andbeam-steering to each of the antenna elements 33, 35, 37, 39. Theantenna 31 may be operated over a relatively narrow phase-shift range ora wide phase-shift range, or as desired. The beam can be adjustedmanually or automatically, again as desired. It should be noted that themajor difference between the unique AM generation technique, which isthe subject matter of the present invention, and conventionalphasedarray antenna techniques lies not in the hardware but in how thehardware is used.

Two classes of modulated phased-array antenna patterns with relationshipto the four element array can be identified: The first involves thegeneration of a fixed antenna pattern shape which is moved about inspace over a relatively small angle by using a suitable modulation inputto the phase-shifter of the antenna. This operation is called thebeam-steering mode because the general shape of the antenna beam remainsfixed as it is moved or steered by the modulator or modulating signal.The second class is much more versatile and more complex; this is calledspatially selective amplitude modulation (SPASAM). In this mode theentire static antenna pattern is modulated so that a new antenna patternwill appear at each instant of time as the modulation signal which isvaried through its entire range. This complex modulation'transferfunction of the antenna can be determined in terms of staticantenna-pattern measurements and will be described in conjunction withthe discussion of the beam-steering mode function.

Referring to FIGS. 3a, 3b, and 3c, terms 4), through define the phasecontribution of the physical spacing of each element of the antenna andis a function of the spatial angle. The terms qS also include DC phaseshifts introduced to steer the antenna beam in space. The deviation 41,,through (b represents only those contributions caused by the ACmodulating signal applied to the antenna. Two cooperating phased-arrayscan be simulated by driving (1) and 4 with an identical DC bias signalfor steering and allowing 4) and (1);, to be identical and d) and 4: tobe zero; this would simulate two cooperating phased-arrays, eachoperated in the beamsteering mode as illustrated in FIG. 2. A singlelarge antenna can be simulated for modulation purposes by driving 11),,through (p with the proper bias and allowing (b and 4);, to be equal andd) and 4: to be zero. This full-size antenna simulation demonstrates thebeamsteering mode.

The large aperture simulation for the general modulation transferfunction mode can be simulated by allowing 4 through 111 to assume anydesired set of nonzero values and to be whatever phase functions thatare necessary to shape the beam and to steer it in the desireddirection. The unique antenna transfer function device can producespectra that are similar to an AM suppressed carrier signal.

It should be noted that the SRD phase-shifters used in the four elementarray, illustrated in FIG. 2, can be used as the phase-shifters in themultiple element array illustrated in FIG. 1.

Many different beam modulation techniques are possible. Beam position,beam shape and the number of beams all can be changed to producemodulation.

What is claimed is:

1. An amplitude modulated phased-array antenna system comprising:

a. a plurality of antenna elements forming an aperture;

b. said plurality of antenna elements each having phase shifting deviceconnected thereto;

c. a means for steering a radio beam operatively connected to each ofsaid phase-shifting device said beam steering means supplying abeam-steering signal to each one of the said plurality of antennaelements; and

d. a means for generating a space-time variable nonlinear phase functionsignal across the aperture of said plurality of antenna elements.

2. The device recited in claim 1 wherein said beamsteering means is a DCbias signal means.

3. The device recited in claim 1 wherein said means for generating aspace time variable non-linear phase function is a modulator to generatea selected modulated phase control signal to each of said phase shiftingdevices simultaneously with the application of said beam-steeringsignal.

4. The device recited in claim 3 wherein each of said selected modulatedphase control signals are AC signals with the same time waveform.

5. The device recited in claim 3 wherein the generated beams of energyfrom each one of a plurality of antenna elements are varied by varyingmeans for generating a space time variable non-linear phase functionsignal over a period of time.

6. The device recited in claim 1 wherein said plurality of antennaelements of the phased-array antenna system comprises:

a. a first antenna element;

b. a second antenna element;

0. a third antenna element;

d. a fourth antenna element; and

e. each of said antenna elements being nonuniformally spaced along areflecting means.

7. The device recited in claim 6 wherein the system further includes aRF signal generating means operatively connected to each one of saidantenna elements to supply an output signal to each of said antennaelements.

8. The device recited in claim 7 wherein said RF signal generating meansfurther comprises a means for generating beam-steering control and ameans for controlling the modulation phase simultaneously to each one ofsaid first, second, third, and fourth antenna elements located on saidreflecting means wherein said DC signal provides a beam-steering signalto each one of said antenna elements.

9. The device recited in claim 8 wherein each of said antenna elementsis a dipole element wherein said second antenna element is spacedone-half a wave-length from said first antenna element wherein saidfirst antenna element is spaced three and one-half wavelengths from saidthird antenna element and four wavelengths from said fourth antennaelement.

1. An amplitude modulated phased-array antenna system comprising: a. aplurality of antenna elements forming an aperture; b. said plurality ofantenna elements each having phase shifting device connected thereto; c.a means for steering a radio beam operatively connected to each of saidphase-shifting device said beam steering means supplying a beam-steeringsignal to each one of the said plurality of antenna elements; and d. ameans for generating a space-time variable non-linear phase functionsignal across the aperture of said plurality of antenna elements.
 2. Thedevice recited in claim 1 wherein said beam-steering means is a DC biassignal meanS.
 3. The device recited in claim 1 wherein said means forgenerating a space time variable non-linear phase function is amodulator to generate a selected modulated phase control signal to eachof said phase shifting devices simultaneously with the application ofsaid beam-steering signal.
 4. The device recited in claim 3 wherein eachof said selected modulated phase control signals are AC signals with thesame time waveform.
 5. The device recited in claim 3 wherein thegenerated beams of energy from each one of a plurality of antennaelements are varied by varying means for generating a space timevariable non-linear phase function signal over a period of time.
 6. Thedevice recited in claim 1 wherein said plurality of antenna elements ofthe phased-array antenna system comprises: a. a first antenna element;b. a second antenna element; c. a third antenna element; d. a fourthantenna element; and e. each of said antenna elements beingnon-uniformally spaced along a reflecting means.
 7. The device recitedin claim 6 wherein the system further includes a RF signal generatingmeans operatively connected to each one of said antenna elements tosupply an output signal to each of said antenna elements.
 8. The devicerecited in claim 7 wherein said RF signal generating means furthercomprises a means for generating beam-steering control and a means forcontrolling the modulation phase simultaneously to each one of saidfirst, second, third, and fourth antenna elements located on saidreflecting means wherein said DC signal provides a beam-steering signalto each one of said antenna elements.
 9. The device recited in claim 8wherein each of said antenna elements is a dipole element wherein saidsecond antenna element is spaced one-half a wave-length from said firstantenna element wherein said first antenna element is spaced three andone-half wavelengths from said third antenna element and fourwavelengths from said fourth antenna element.